Shielded grid tube



W. L. KRAHL Jan. 12, 1932.

SHIELDED GRID TUBE 2 Sheets-Sheet Filed NOV. 2. 1927 llllLlK Jan. 12, 1932. w, KRAHL 1,840,365

SHIELDED GRID TUBE Filed Nov. 2. 1927 2 Sheets-Sheet 2 m INVESTOR 45 HHIHm 49 #444752 lows KfiW/L 6 w Wx RNEY Patented Jan. 12, 1932 UNITED STATES PATENT orrlce WALTER LOUIS KRAEL, OF MONTOLAIB, NEW JERSEY, ABSIGNOB TO ABO'IUBUB RADIO TUBE COMPANY, OF NEWARK, NEW JERSEY, A OOBPORATION OI DELAWARE SHIELDED GRID TUBE Application filed November 2, 1927. Serial Io. 280,448.

I This invention relates to double grid tubes,

and es ecially to tubes adapted to be energized y alternating or fluctuating current from a commercial source.

In all three-element tubes, the amplifica tion is limited by the inter-element capacity,

especially thatcapacity which is caused by the close proximity and parallel relation of the grid and plate. The capacity between the .0 grid and plate permits the fluctuations of the plate circuit to cause fluctuations of the grid circuit which in turn are amplified in the plate circuit, and an'oscillating condition is set up which is detrimental to the efliciency Is of the tube, as it produces howling and other noises and tends to unbalance the radio receiving set.

It is one of the objects of the present invention to provide a means to reduce the ca- 1 '2 pacity effect between the plate and grid elements of a vacuum tube.

Another object of the invention is to provide a means to completely shield the grid of a vacuum tube from variations of potential on the plate by the addition of an element interposed between the grid and plate.

Another object of the invention is to provide a means to reduce the capacity effect between the grid and plate elements of a vac- 39 uum-tube, which means is connected inside of the tube so that no additional connection is needed on the outside thereof.

Another object of the invention is to pro- 7 vide a means to reduce inter-element capacity 35 in a thermionic tube, without consuming any of the energy of the tube.

Another object of the invention is to provide a thermionic tube having means to reduce the capacity efiects between the plate and grid elements thereof, all of the elements being mounted solely upon the conductors for the elements.

Other objects of the invention, such as relating to methods of construction. and economies of manufacture, will be apparent from a single press in the tube.

as the description of the invention proceeds.

The invention has been illustrated in the accompanying drawings, in which I Fig. 1 is a perspective view of a thermionic tube embodying the invention, certain portions of the structure being broken away to show the construction more clearly;

Fig. 2 is a perspective view of a portion of a tube showing a modification of the inven tion as illustrated in Fig. 1; and

Fig. 3 is a circuit diagram showing the manner in which the tube is connected in a circuit.

In my applications for vacuum tubes, Ser. No. 140,245, filed October-8, 1926, and power operated vacuum tube construction and circuit therefor, Ser. No. 156,946, filed December 24, 1926, I have shown and described a thermionic tube adapted to operate on alternating or fluctuating current from commercial sources. This tube comprises a tubular cathode having a single heater element positioned axially thereof and separated therefrom by vacuous space, the cathode and heater element being connected together at the top of the tube, and the cathode therefor forming a return path for the heater current. The usual grid and plate have also been described as supported in the usual manner 75 In the present application, I have also shown a similar relationship between cathode, heater, grid and plate, with the exception of an additional grid interposed between the control grid and plate.

In Fig. 1,-a glass envelope 10 is shown mounted upon a base 11 of a standard design which carries terminals 12, 13, 14 and 15, spaced apart in a standard manner so that the tube may fit into a standard four terminal socket. A single press 16 is providedinside of the envelope 10, which press supports all of the elements in the tube. The plate or anode comprises a cylindrical shell 17 welded to a support rod 18 which is sealed in the press 16 and forms not only a support for the plate, but the conductor therefor, as well. A grid 19, which may be formedv of a spiral coil of wire, as is the usual practice, is welded to a support rod 20 which in turn is welded at its lower end to the conductor 21 which is sealed in the press 16. The conductor 21 forms a support for the grid 19 and also acts as the conductor therefor. A support rod 22 may also be welded to the opposite side of the grid coil to make the grid construction more rigid.

The cathode as described in the above mentioned applications,comprises a tube23which extends somewhat above and below the plate 17 and is treated intermediate the points opposite the ends of the plate 17 by an electron emitting oxide as at 24. This cathode has a high efliciency doubtless, because the electron emitting surface is large producing high denisty of electron emission and low impedance. The lower end of the cathode 23 may be attached to a support rod 25 by a band 26 which may surround the end of the cathode. The rod 25 forms a support for the lower end of the cathode and also a conductor for the cathode and one sideof the heating circuit, as will be hereinafter explained. The heating element comprises a single straight filament 27 of high resistance material, preferably carbon, which may be attached to a rod 28 which forms, similarly to the rods 18, 21 and 25, the supportand conductor for the lower end of the filament.

Means is provided to rigidly support the upper ends of the plate, grid, cathode and heating element and to such end a rod 29 is welded to the support 25 and extends at right angles thereto to a point spaced outside of the plate where it is bent upwardly to a point above the top of the plate and then inwardly a short distance for a purpose to be hereinafter described A glass bead 30 is sealed on the vertical portion of the rod 29 and forms a rigid support to insulatingly connect the upper ends of the various elements. The upper end of the plate 17 may be tied to the glass head 30 by a short angular rod 31 which may be sealed in the glass bead and welded to the upper end of the support rod 18. Another angular support rod 32 may also be sealed in the glass head 30 and its outer end welded to the upper end of the support rod 20, thereby forming a rigid support for the upper end of the grid. The upper. end of the cathode 23 may be secured to the vertical portion of the rod 29 by a strip 33 which may surround the upper end of the cathode and be welded thereto and which may have its outer end welded to the support 29. The filament 27 may be attached at its upper end to a short conductor 34 which may be welded to a spring 35 extending horizontally adjacent the strip 33, to the upper inturned end of the rod 29, to which it may be welded. The spring 35 forms a means to compensate for variations in length of the filament due to temperature changes and, therefore, maintains the filament straight and taut inside of the cathode.

It will thus be evident that the plate, grid, cathode and filament are rigidily supported in spaced relation at both the upper and lower ends thereof, the four conductors being the sole support for these elements. It will also be noted that the filamentand cathode are connected together electrically at the upper end by means of the spring 35 and strip 33 and current ma therefore, flow in series through the con uctor 28, filament 27, spring 35, strip 33, cathode 23 and conductor 25.

All of the above described structure has been illustrated in the applications above referred to, and also in my application entitled Improvements in tube construction, Serial No. 229,881, filed Oct. 31, 1927, wherein the four supporting conductors were illustrated, and the means to reduce the capacity between the grid 19 and the plate 17, which forms the subject matter of the present in vention, will now be described.

This means comprises an auxiliary grid 36,

which may be formed in any desired manner,

37a sealed into the press 16, thereby forming a support for the auxiliary grid 36 and also a connection to the cathode 23, through the wire 37 b which is connected to the lead coming from the conductor 25. The upper end of the rod 37 may be welded to an angle support 38 which may be sealed in the glass head 30 similarly to the supports 31 and 32, which construction affords rigid means of supporting the auxiliary grid 36 in spaced relation between the grid 19 and the plate 17. If desired, another support rod 39 may be welded to the opposite side of the coil 36 to maintain the wires in spaced relation to each other.

In the ordinary vacuum tube when an additional grid is used in order to reduce the capacity between the control grid and anode,

it has been found necessary to maintain a potential on the auxiliary grid, or in other words, to connect the auxiliary grid to a point more positive than the cathode, and a potential of as high as 45 volts has been advocated face at a constant potential, I have found that the auxiliary grid may be connected directly to the cathode with excellent results.

The characteristics of one tube embodying my invention were found to be as follows:

Amplification constant equal to 30.0; Plate resistance equal to 60,000 ohms; Mutual conductance equal to 500 micromhos when the filament electromotive force was ual to 15.0 volts alternating current; the p ate potential was equal to 90 volts; and the grid) potential was equal to 1.5 volts, (negative Wires 40, 41, 42 and 43 may be used in the usual manner to connect supports 18, 25, 28 and 21 to the base terminals 12, 13, 14 and 15 respectively.

' It will be noted that inasmuch as the conductors 18 for the P ate, and 21 for the grid are parallel, and wires 40 and 43 extend down through the base of the tube, a slight capacity effect is. produced in these conductors and wires which may have an efiect upon the operation of the tube, even though extremely slight. If it is desired to eliminate this effect, the modification shown in Fig. 2 may be used. In this modification the elements of the tube are exactly the same as those shown in Fig. 1 with the exception that the auxiliary grid is made longer so as to shield, as far as possible, the supports for the control grid, and the elements are spaced apart in a similar manner, but instead of the conductor 21 extending down from the grid support 20, to the press 16, I have,eliminated this conductor and have provided instead, a conductor 44 which may be welded to the grid conductor 20, and sealed in a press 45 formed in the top of the glass envelope 10. In place of the support 21 for the control grid I have provided a quartz glass tube 54 which may be supported upon a short rod 55 sealed in the press 16. The upper end of the glass tube may be bent 45 inwardly and upwardly to receive the lower end of the rod 20. In this manner, the control grid is supported and a connection is made thereto by means of a cap 46 at the top of the tube, thus eliminating the possibility of capacity being set up between the plate and grid conductors. The filament cathode and plate conductors are connected to three of the base terminals similarly to the manner shown in Fig. 1.

The tube may be connected in a circuit in a manner well known in the art, such connection being indicated in Fig. 3. The tube terminals 13 and 14 may be directly connected to the secondary 47 of a transformer 48, the primary 49 of which is connected to a source 50 of-alternating current.

. The grid circuit may be connected from the terminal 15 and mayinclude the input circuit and a small battery 51 to place the usual 65 negative potential on the grid. The output circuit may be connected as usual to the plate terminal 12 and may include the usual B- batterg lf2for maintaining the plate potential. e auxiliary grid 36 is indicated between the grid and anode and is shown connected inside the tube to the cathode and filament circuit at 53.

With a tube construction as indicated in Fig. 1, I have found that the inter-element capacity may be reduced from about 10 to 13 micromicrofarads to 3 or 4 micromicrofarads while by using the construction indicated in Fig. 2, this capacity may be still further reduced.

While I have illustrated and described the invention as particularly adapted to tubes energized by alternating current, it is evident that direct current tubes employing a high efiicienc cathode may also be used in connection with the auxiliary grid, and various other changes in the circuit and modifications of the structure of the tube may be resorted to without depearting from the spirit of the invention, so that I do not, therefore, desire to limit myself to the s ecific construction shown and described, ut to interpret the invention broadly within the scope of the appended claims.

Having thus described my invention, what I claim is:

1. A thermionic tube comprising an anode, a grid, a cathode, a heater for said cathode, an auxiliary grid between said grid and anode, supporting conductors attached to said anode, grid, cathode, heater and auxiliary grid respectively, a supportin bead, means connected to the upper end 0 said cathode. and heater for maintaining the same in spaced relation, said means being in the form of a conductor extending through said bead to support the same and being connected to the supporting conductor attached to said cathode, and means each supported by said bead secured to the upper end of said anode, grid and auxiliary grid respectively for maintaining these respective elements in spaced position with respect to each other.

2. A thermionic tube comprising an anode,

a grid, a cathode, a heater for said cathode, an auxiliary grid between said grid and anode, supporting conductors attached to said anode, grid, cathode, heater and auxiliary grid respectively, a supporting bead, means connected to the upper end of said cathode and heater for maintaining the same in spaced relation, said means being in the form of a conductor secured to said upper ends and to each other, said means extending through said bead to support the same and being connected to the supporting conductor attached to said cathode, a support attached to the upper end of said anode, a support attached to the upper end of said grid and a support attached to the upper end of said auxiliary grid, said three last-mentioned supports having their ends embedded in said bead and serving to maintain the elements to which they are attached in spaced relation.

3. A thermionic tube comprising an envelope, a press within said envelope, an anode, a grid, a cathode, a heater for sald cathode, an auxiliary grid between said grid and anode, a conductor attached to said anode, a conductor attached to. said grid, a conductor attached to said cathode, a conductor attached to said heater, a conductor attached to said auxiliary grid, said conductors being sealed in said press providing a sole support for sald anode, grid and auxiliary grid, a conductor attached within the press to said auxiliary grid being connected electrically to the conductor support to said cathode.

4. A thermionic tube comprising an envelope, a press within said envelope, an

anode, a grid, a cathode, a heater for said cathode, an auxiliary grid between said grid and anode, a conductor attached to said anode, a conductor attached to said grid, a conductor attached to said cathode, a conductor attached-tosaid heater, a conductor attached to said auxiliary grid, said conductors being sealed in said press providing a sole in spaced position, a supporting rod attached support for said anode, grid and auxiliary grid, a conductor attached to said auxiliary grid being connected electrically to the conductor support to said cathode, a' bead, a conductor attached to the upper end of said heater and cathode and extending through said head and being secured to .the conductor supporting said cathode, a support connected to the upper end of said anode, grid and auxiliary grid respectively, said three lastmentioned supports having their ends embedded in said bead.

5. A thermionic tube comprising a cathode, an anode, a heater for said cathode, a plurality of grids, a supporting rod for said cathode, a supporting rod for said anode, a supporting rod for said heater and a supporting rod for each of said grids, a press, said supporting rods extending downwardly from their respective elements into said press, a bead, means for supporting said bead in the tube,

and means supported by the bead for .maintaining the upper ends of the cathode, anode, heater and firids in spaced relation with respect to eac other.

6. A thermionic vacuum tube of the enclosed glass type having a press, a cylindrical cathode, a central heater extending'through said cathode, a cylindrical anode surrounding saidcathode and a plurality of cylindrically shaped grids concentrically mounted around said cathode. and within said anode, a supporting rod secured to each of said grids .and extending into the press, a bead supported from the press, a supporting rod attached to the upper end of each of said firstmentioned rods and extending into the bead for maintaining the upper end of said-grids 

